Wedge,
Your power supply is weak, but lets try a few things first. (You should look for a better power supply with at least 28a in the 3.3v RAIL and a Max Combined Wattage of at least 200W for the future.)
The nForce2 Chipset with integrated graphics has only one AGP Bus that auto senses an AGP Video Card. There is no option in the BIOS for turning off onboard graphics since it happens automatically. (You cannot have two video cards on one bus.) But that is not the case here since that motherboard does not have onboard graphics. (None of the Ultra 400s do.)
RAM: 2x(6506)512 MB PC 2700 DDR 333 DIMM
I am going to start with the RAM being the possible problem. If your RAM is not nForce Approved there is a good change you will just hear the Hard Drives Spin up but it does not boot.
Try removing all Drives, PCI Cards, and one stick of RAM. Try to see if you can get to the BIOS with one stick of RAM. If not try with just the other stick of RAM. (It would really help knowing the brand of RAM you are using.)
That board will support all GeForce Cards. If you have a TnT2 or older, it will not be supported.
Take Care,
Richard